This invention relates to sleep monitors, used e.g., in therapy, diagnosis, or research, which differentiate various sleep and wakeful states.
Normal individuals experience distinct sleep states. One important sleep state, REM sleep, is characterized by rapid eye movements, small muscle twitches, and the absence of other body movements. The other sleep state, non-REM (NREM) sleep is subdivided into four grades or states (I-IV), stage I being the most shallow (least restful or refreshing) and stage IV being the deepest.
Monitoring and individual's sleep state is important for diagnosising sleep disorders. It is also important for diagnosising and following response to treatment of depression (affective disorder) in which REM latency is significantly reduced.
It is estimated that 8-15% of the adult U.S. population have frequent complaints about sleep quality and quantity, and 3-11% use sedative hypnotic drugs for these complaints. Sleep disorders can be debilitating. They are expensive and difficult to diagnose and treat. For diagnosis, the patient's sleep state may be monitored to determine the pattern and duration of various sleep states. Sleep is qualitatively and quantitatively evaluated by measuring the electrical signals produced by brain and muscle activity, using electrophysiological techniques and instruments. A widely used technique for this purpose (described in Rechtshaffen and Kales, eds., A Manual of Standardized Terminology, Techniques, and Scoring System For Sleep Stages of Human Subjects, Wash. D.C. U.S. Gov't. Print Off. Public Health Service) involves simultaneously and continuously measuring electroencephalographic (EEG) data--signals derived primarily from the cortex of the brain and sometimes referred to as an electrocortigram (ECoG)--along with an electromyogram (EMG) signal which monitors muscle activity, generally from one of the muscles of the lower jaw, together with left- and right-eye electrooculogram (EOG) signals produced by eye movement. These EEG, EMG, and EOG signals are conventionally recorded on a multi-channel physiological recorder (sometimes referred to as a polysomnograph). A skilled technician, using standardized criteria for evaluating such recordings, grades each period of the recording as awake, NREM state I to IV sleep, or REM sleep, to produce a sleep profile of the type of sleep as a function of time. The technician will then determine the proportion of the total sleep period spent in each of the grades of NREM sleep and in REM sleep.
The above method, which is the "standard" in sleep research, usually involves the use of a large number of (e.g. ten) electrodes pasted and taped to the subject's body. These electrodes and the wires connecting them to the polygraph recorder cause discomfort and restricted movement. It can be difficult to obtain a time series study on a subject because of the discomfort involved from the associated measurement apparatus.
The above method also requires costly equipment and skilled labor. A standard sleep investigation can cost in the neighborhood of $1000 per night, and it can last for several nights. There may be long waiting lists for sleep evaluation labs.
In view of the difficulties with existing sleep evaluation techniques, many patients who present with sleep disorders are not tested with those techniques, and are treated with sedative-hypnotic drugs without detailed evaluation.
Photographic techniques also have been used to evaluate sleep state. Hobson et al. (1978) Science 201: 1251-1255 measured the mobility of sleeping subjects photographically and predicted transitions between NREM and REM on the premise that major body posture shifts occur immediately preceeding and following REM sleep. Hobson et al. suggest that " . . . [P]ostural immobility, easily detectable in time lapse photographic data, could by itself provide a simple quantitative read-out of the state of the brain oscillator controlling the REM-NREM sleep cycle." Time lapse photography is suggested as a means of conducting field studies of sleep behavior.
Aaronson et al. (1982) Arch. Gen. Psychiatry 39: 330-335 report the use of time-lapse video recording to monitor sleep state, based on the knowledge that major body movements are known to occur predominantly before and after recurrent episodes of REM sleep, and the longest periods of immobility are associated with non-REM episodes. Sleep latency and REM onset were predicted from such major movements. Evidence of small body movement specific to REM sleep was also observed.
Kayed et al. (1979) Sleep 2 (2): 253-260 disclose an antioculographic sleep monitor (AOGM) system for recording eye movement, body movements and electromyogram (EMG) signals to monitor muscle activity. In addition to the EMG signal, the system includes a piezo-ceramic transducer attached to the eyelid to sense eye movements and a second transducer attached to an index finger joint to sense body movement. Analog signals from each of the sensors are recorded on tape, and the tapes are analyzed and scored by standard criteria. At p. 260, Kayed et al. say,
"The introduction of miniature transducers that can be applied directly on the eyelid provides an easy method for recording eye and body movements and makes simultaneous eye and body movement monitoring possible. The various combinations of these two parameters await further attention, and the study of their temporal relationships may provide interesting information to sleep phenomenology. The use of submental EMG helps to confirm the transition from non-REM to REM; however, further experience may prove that its use is not mandatory."